In the manufacture of lightweight paper grades, such as newsprint and fine paper, the web is dried on a series of steam-heated drying cylinders. To dry the web, it is pressed directly onto the cylinders by a series of tensioned, permeable dryer fabrics or felts.
In a conventional double-felted, two tier dryer section, the wet web passes from one cylinder to the next in a generally serpentine fashion through lengthy zones in which the web is unsupported, called "open draws." The majority of water vapor that leaves the web during drying is released in these open draws. The use of open draws, however, is associated with numerous problems relating to runnability and web quality. First, the web tends to flutter in the open draws. Second, dryers utilizing open draws typically require threading ropes in order to thread a tail of the web through the dryer sections. Finally, the use of open draws results in web quality problems. The web, which is unrestrained in the cross-machine direction in the open draws, tends to shrink, particularly at the edges, as water evaporates from the web. This shrinkage is non-uniform, and results in such web surface defects as curl, cockle, and graininess.
Some of the problems with web flutter and sheet shrinkage have been solved by using single-tiered dryer sections alone or in combination with double-tiered dryer sections. In a single-tiered dryer section, such as those in the BelRun.TM. and Bel-Champ.TM. dryers manufactured by Beloit Corporation of Beloit, Wis., the web moves from one drying cylinder to another by passing around a transfer roll, supported by a felt. Thus, in the zones between the cylinders and transfer rolls, the web is directly supported by the felt. This is known as a "closed draw." The use of a closed draw has improved runnability by reducing web flutter and eliminating the need for threading ropes. Drying of both sides of the web is achieved by passing the web between alternating top-felted and bottom-felted sections. The web is transferred between these sections using a unique felt arrangement, such as that shown in U.S. Pat. No. 4,934,067.
In the single-tiered dryer sections, sheet restraint is provided using the combination of felt pressure against the web on the cylinders, and vacuum pressure against the web on the transfer rolls between the cylinders, caused by the formation of a partial vacuum at the surface of the transfer rolls. Transfer rolls designed to form such a vacuum are appropriately known as vacuum transfer rolls. The use of such vacuum transfer rolls in all the drying sections of a drying apparatus has been shown to reduce cross-directional web shrinkage by 60 to 80 percent.
The use of single-tiered dryer sections has not, however, fully solved the runnability or web shrinkage problems. Occasionally, the wet web will separate from the felt. Such separation generally occurs in the zone, or distance, between the dryer cylinder and the vacuum transfer roll. This zone, or distance, is known as the "down run." Although the "down run" is generally short, less than 18 to 20 inches, and represents a direct path from the dryer cylinder to the vacuum roll, the web can separate from the felt due to adhesive forces between the web and the dryer cylinder on the cylinder surface. This separation is often related to incorrect or uncontrolled chemistry in the stock preparation, forming or pressing areas, or incorrect or uncontrolled airflows.
To solve the web separation problem, various sealing mechanisms have been combined with the vacuum transfer rolls so that a partial vacuum is created in the region between adjacent cylinders bounded by the felt extending between the two cylinders and the transfer roll. This region is known as the "pocket." This partial vacuum against the felt on the side opposite to that which carries the web, urges the web against the felt in the down runs. Such a design is shown, for example, in U.S. Pat. No. 4,876,803 ("the '803 patent"). The '803 patent discloses a vacuum transfer roll with a perforate shell in combination with sealing means comprising a wedge-shaped box disposed within the pocket. As stated above, a partial vacuum is created in the pocket, causing the web to be urged against the felt, thus resisting separation in the down run.
Such a design has been found to be more effective in reducing web separation than the use of a conventional dryer section with the vacuum transfer rolls located the usual two to six inches from the dryers. The vacuum level achieved in the pocket areas, however, is generally insufficient to completely restrain the web from cross-directional shrinkage. The maximum practical vacuum that can be achieved in the pocket areas is limited by felt deflection, leakage from the sealing means, felt wear, operating costs involved in creating the vacuum, and the tendency for broke to accumulate on the sealing means, resulting in damage, such as burning, to the felt.
In addition, the partial vacuum applied against the felt in the "down-run" is applied when the web and felt are in a generally flat orientation. Much higher vacuum levels are required to restrain the web when it is flat than is required when the web is wrapping around the vacuum transfer roll. Thus, although the use of vacuum transfer rolls combined with pocket sealing means, such as vacuum transfer rolls having external vacuum chambers, to create a partial vacuum against the felt in the down run has reduced web separation and thus improved runnability, it is not particularly effective in preventing cross-directional shrinkage.
It has been proposed to reduce cross-directional web shrinkage by reducing the length of the down run, or distance between the vacuum transfer roll and its adjacent drying cylinders. By reducing the length of the down run, the amount of time the web is in a flat orientation is reduced. Such a reduction in down run length has been accomplished by the use of pivoting vacuum transfer rolls. Pivoting vacuum transfer rolls, such as those disclosed in U.S. Pat. No. 4,905,379, are capable of reducing the down run length during operation of the dryer by reducing the gap between the dryer and following vacuum transfer roll to a distance of no more than about 0.5 inches to about 1 inch. However, the use of pivoting vacuum rolls does not result in the improved runnability obtained from the use of vacuum rolls with external vacuum chambers to create a partial vacuum in the pocket against the side of the felt opposite that on which the web is carried.
Thus, there is a need for a paper web drying apparatus that exhibits both improved runnability and improved cross-directional web shrinkage restraint.